The thermodynamic equilibrium of the reservoir fluids may end up with the pressure difference between fluid phases (called capillary pressure) due to difference in fluids interaction with rocks:

P_{cow} = p_o - p_w

P_{cog} = p_o - p_g

P_{cgw} = p_g - p_w

where

Some engineering applications make fair use of the concept of Average phase pressure :

p = \frac{1}{3} \, ( p_w + p_o + p_g )

p = \frac{1}{2} \, ( p_w + p_o )

p = \frac{1}{2} \, ( p_o + p_g )

p = \frac{1}{2} \, ( p_w + p_g )

See also

Natural Science / Physics /  Fluid Dynamics / Percolation

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